OVERVIEW

The CQIQC/Toronto Quantum Information Seminar - QUINF - is held
roughly every two weeks to discuss ongoing work and ideas about
quantum computation, cryptography, teleportation, et cetera. We
hope to bring together interested parties from a variety of different
backgrounds, including math, computer science, physics, chemistry,
and engineering, to share ideas as well as open questions.The CQIQC/Toronto
Quantum Information Seminar - QUINF - is held roughly every two
weeks to discuss ongoing work and ideas about quantum computation,
cryptography, teleportation, et cetera. We hope to bring together
interested parties from a variety of different backgrounds, including
math, computer science, physics, chemistry, and engineering, to
share ideas as well as open questions.

Upcoming
Seminars

June 28,
11:10 a.m.
Room 210

Savannah Garmon, University of Tokyo Bound state influence on long-time non-exponential decay in open
quantum systems

We study the long time non-exponential decay in an open quantum system
in which a bound state approaches the energy continuum as the system
parameters are varied. We find that at least two regions exist yielding
different power law decay behaviors; we term these the long-time near
zone and the long-time far zone. In the near zone the survival probability
falls off according to a t^-1 power law, while in the far zone it
falls off as t^-3. The timescale T_Q separating these two regions
is inversely related to the gap between the bound state energy and
the continuum threshold, hence when the bound state is absorbed into
the continuum for special parameter values, this time scale diverges
and the survival probability follows the enhanced t^-1 power law even
on asymptotic scales.

Howard Wiseman, Griffith University 100 years of quantum jump theory: Is there anything new
to say for a 2-level atom?

The famous "quantum jump" of an atom from one
level to another was first postulated by Bohr in 1913, nearly
100 years ago. Can there still be anything interesting to
say about quantum jumps in a two-level atom today, 100 years
later? Of course my answer is "yes", as I will
be talking about two recent papers in this area [1,2]. The
most recent [2] proposes experiments to definitively prove
that quantum jumps are not due to emission of a photon,
but rather to detection. Such a test is possible because
different types of detection lead to different types of
jumps. The other [1] shows that this flexibility in causing
jumps of different types can always be used creatively to
make an atom jump between a two fixed states, although in
general these are NOT orthogonal.

Quantum computers are among the most promising applications
of quantum-enhanced technologies. Quantum effects such as
superposition and entanglement enable computational speed-ups
that are unattainable using classical computers. Blind quantum
computing shows an additional fundamental advantage of quantum
over classical computation: a computation can be made private.
Using the framework of measurement-based quantum computing,
blind quantum computing enables a nearly-classical client
to access the resources of a more computationally-powerful
quantum server without divulging the content of the requested
computation. Here, we demonstrate the first experimental
version of this protocol using polarization-entangled photonic
qubits. We show various blind one- and two-qubit gate operations
as well as blind versions of the Deutsch's and Grover's
algorithms. Further, we address the question if quantum
computations can be verified by classical entities which
are not able to compute the results themselves. When the
technology to build quantum computers becomes available,
blind quantum computing and the verification of quantum
computations will become an important feature of quantum
information processing.

May 17,
11:10 a.m.
Room 210

Roman Krems (The University of British Columbia) Collective excitations of molecules trapped on an optical
lattice

Molecules trapped on an optical lattice represent a unique,
controllable many-body system which can be used to study
dynamics of collective excitations in new regimes. I will
discuss the rotational excitations of molecules on an optical
lattice leading to rotational Frenkel excitons. Apart from
solid hydrogen, there is no other natural system that exhibits
rotational excitons. The rotational excitons have unique
properties that can be exploited for tuning non-linear exciton
interactions and exciton-impurity scattering by applying
an external electric field. I will show that this can be
used to explore the competing role of the dynamical and
kinematic exciton-exciton interactions in excitonic energy
transfer and to study quantum localization in a dynamically
tunable disordered potential.

The rotational excitons can also be used as a basis for
quantum simulation of condensed matter models that cannot
be realized with ultracold atoms. In particular, I will
discuss the possibility of engineering the Holstein, breathing-mode
and Su-Schrieffer-Heeger polaron models with polar molecules
on an optical lattice. I will discuss the phase diagram
of a polaron model with mixed breathing-mode and Su-Schrieffer-Heeger
couplings and show that it has two sharp transitions, in
contrast to pure models which exhibit one (for Su-Schrieffer-Heeger
coupling) or no (for breathing-mode coupling) transition.
I will show that ultracold molecules trapped in optical
lattices can be used to study precisely this mixed Hamiltonian,
and that the relative contributions of the two couplings
can be tuned with external electric fields, which brings
the possibility of observing the polaron transitions within
reach of up-coming experiments. Time permitting, I will
also discuss dipole blockade of microwave excitations in
ensembles of trapped molecules and how it can be used to
create quantum phases of interacting spin-1/2 particles
without crossing phase transitions.

Quantum information science addresses how uniquely quantum
mechanical phenomena such as superposition and entanglement
can enhance communication, information processing, and precision
measurement. Photons are appealing for their low-noise,
light-speed transmission and ease of manipulation using
conventional optical components. However, it has been very
difficult to achieve the necessary two-qubit operations
since the physical interaction between photons is much too
small. In a breakthrough, Knill, Laflamme, and Milburn (KLM)
showed that effective nonlinear interactions can be achieved
using only linear optical elements, auxiliary photons, and
measurement. Inspired by the KLM approach, a number of quantum
logic gates using heralded photons and event postselection
have been proposed and demonstrated. Furthermore, optical
quantum circuits combining these gates have been demonstrated
. We experimentally demonstrate a two photon quantum gate
(controlled-NOT gate) based on the KLM approach. This result
confirms the first step in the original KLM recipe
for all-optical quantum computation, and should be useful
for on-demand entanglement generation and purification.
Our other recent progress on linear optics quantum circuit
will also be introduced.

Optical waveguides provide tight confinement of light over
extended lengths which are ideal for nonlinear optical interactions.
I will describe the use of a various types of waveguides
including photonics crystal fibers and silicon-based nanowaveguides
for quantum information applications. For example, we use
hollow-core photonics band-gap fibers filled with Rb atoms
to enable nonlinear interactions down to the few-photon
level. In addition, we can use dispersion engineering in
glass and silicon nanowaveguides to produce quantum-correlated
photons, frequency translation of quantum states, and photon
shaping.

Our sense of smell is extraordinarily good at molecular
recognition: we can identify tens of thousands of odorants
unerringly over a wide concentration range. The mechanism
by which this happens is still hotly debated. One view is
that molecular shape governs smell, but this notion has
turned out to have very little predictive power. Some years
ago I revived a discredited theory that posits instead that
the nose is a vibrational spectroscope, and proposed a possible
underlying mechanism, inelastic electron tunneling. In my
talk I will review the history and salient facts of this
problem and describe some recent experiments both on fruit
lies and on humans that go some way towards settling the
question.

A dissipative environment usually transforms a quantum
superposition into a classical state. Recent advances in
superconducting circuits--the development of robust quantum-noise-limited
microwave amplifiers and quantum bits with lifetimes in
excess of 100ms--have enabled the use of quantum feedback
to actively suppress decoherence. We discuss experiments
in which microwave pulses alter the circuit environment
to autonomously cool the system to any coherent superposition
of ground and excited states. In addition, we also realize
weak measurements of the qubit state to implement real-time
feedback. Here, the dominant dephasing is measurement induced
and the information extracted is used to generate Rabi oscillations
which persist indefinitely.

Coupling the surface state of a topological insulator (TI)
to an s-wave superconductor is predicted to produce the
long-sought Majorana quasiparticle excitations. Such Majorana
fermions may be topologically protected from decoherence,
and could play a significant role in solid state implementations
of a quantum computer. A requisite step in the search for
Majorana fermions is to understand the nature and origin
of the supercurrent generated between superconducting contacts
and a TI.
In this talk, I will discuss transport measurements of DC
Josephson effects in TI-superconductor junctions (Bi2Se3-Al)
as the chemical potential is moved from the bulk bands into
the band gap, or through the true topological regime characterized
by the presence of only surface currents. We compare our
results to 3D quantum transport simulations to conclude
that the supercurrent is largely carried by surface states,
due to the inherent topology of the bands, and that it is
robust against disorder. We further find that the supercurrent
is not symmetric with respect to the conduction and valence
bands, and that the Fraunhofer patterns are similar both
within and outside of the topological regime.

Feb. 08,
11:10 a.m.
Room 230

Dylan Mahler (UToronto)Adaptive Quantum State Tomography

Within the past 5 or so years, a number of experiments
have revealed the condensation of polariton quasi-particles
in quasi 2d inorganic quantum well cavities. A polariton
condensate forms when there is a sufficiently high density
of exciations in a material sandwiched between two dielectric
reflectors that spontaneous symmetry breaking occurs and
the exciton gas condenses to form a superfluidic state.
This has opened the door to test a number of novel and fundimental
theories ranging from the BEC to BCS cross over to Hawking
radiation from blackholes. Loosely speaking, the condensation
occurs when all the dipole oscillators in the system are
driven by a common field mode and spontaneously beging to
evolve synchroneously much like the effect of super-radiance.
Our work has focused upon the dynamics of condensate formation
in organic semiconductor-based systems. I will discuss both
the equilibrium and non-equilibrium/steady state regimes
using models based upon one and two dimensional arrays of
organic chromophores. Time permitting, I will discuss our
work on linear arrays of quantum nanorods coupled by a common
surface plasmon mode.

Feb. 1,
11:10 a.m.
Room 210

Eric Bittner (University of Houston)Theory and Models of Bose Condensation of Exciton/Polariton
in Organic Semiconductor Thin-films

Within the past 5 or so years, a number of experiments
have revealed the condensation of polariton quasi-particles
in quasi 2d inorganic quantum well cavities. A polariton
condensate forms when there is a sufficiently high density
of exciations in a material sandwiched between two dielectric
reflectors that spontaneous symmetry breaking occurs and
the exciton gas condenses to form a superfluidic state.
This has opened the door to test a number of novel and fundimental
theories ranging from the BEC to BCS cross over to Hawking
radiation from blackholes. Loosely speaking, the condensation
occurs when all the dipole oscillators in the system are
driven by a common field mode and spontaneously beging to
evolve synchroneously much like the effect of super-radiance.
Our work has focused upon the dynamics of condensate formation
in organic semiconductor-based systems. I will discuss both
the equilibrium and non-equilibrium/steady state regimes
using models based upon one and two dimensional arrays of
organic chromophores. Time permitting, I will discuss our
work on linear arrays of quantum nanorods coupled by a common
surface plasmon mode.

Time-resolving tunneling is a well-recognized controversial
problem. The main difficulty comes from the absence of unambiguous
definition of tunneling time. The question becomes even
more intriguing in many-body systems. First, many body-interactions
during tunneling may delay the electron escape through the
barrier. Second, these interactions can be used to record
the tunneling dynamics. Many-body interactions during tunneling
range from Josefson junction to metal-insulator tunneling,
to electron tunneling from atoms and molecules in strong
infrared laser fields. In latter case the tunneling barrier
is created by the laser field. The corresponding ionization
mechanism is called "optical tunneling" to distinguish
it from the tunneling in static electric fields.
We show how one can use a combination of multicolor (from
infrared to extreme ultraviolet) light fields to time-resolve
optical tunneling in one-electron and many-electron systems.

Oct. 19
Fields Institute,
Room 230

*Please note the room change

Man-Duen Choi (University of Toronto)The Taming of the Shrew - Tricks or Treats with Quantum
Entanglements

I wish to tame the physical quantum entanglements (in disguise
of non-commutative geometry), by means of pure mathematics.
Note that the research work along these lines, has been
proven to be useful to the foundation of abstract quantum
information in the light of (the reality of) quantum computers.
This is an expository talk; no background knowledge of quantum
information will be assumed in this talk.

We will consider the implementation of a symmetric informationally
complete probability-operator measurements (SIC POM) in
the Hilbert space of a d-level system by a two-step measurement
process: a diagonal-operator measurement with high-rank
outcomes, followed by a rank-1 measurement in a basis chosen
in accordance with the result of the first measurement.
We find that any Heisenberg-Weyl group-covariant SIC POM
can be realized by such a sequence where the second measurement
is simply a measurement in the Fourier basis, independent
of the result of the first measurement. Furthermore, at
least for the particular cases studied, of dimension 2,
3, 4, and 8, this scheme reveals an unexpected operational
relation between mutually unbiased bases and SIC POMs; the
former are used to construct the latter. As a laboratory
application of the two-step measurement process, we propose
feasible optical experiments that would realize SIC POMs
in various dimensions. I am looking forward to meet you.

We investigate quantum phases with spinor bosonic gases
using quantum information tools. We show that in finite
quantum spin chains when approaching a quantum phase transition,
the Schmidt gap, i.e. the difference between the two largest
eigenvalues of the reduced density matrix $\lambda_{1},\lambda_{{2}}$,
signals the critical point and
scales with universal critical exponents related to the
relevant operators of the corresponding conformal theory
describing the perturbation from the critical point. Such
scaling behavior allows to identify explicitly the Schmidt
gap as a local order parameter.

Quantum theory is inherently statistical. This entails
repetition of experiments over a number of identically prepared
quantum objects, if one wants to know the "true state"
or the "true value" of the parameter that specifies
the quantum state. In applications, one needs to design
the estimation procedure in such a way that the estimated
value of the parameter should be close to the true value
(consistency), and that the uncertainty of the estimated
value should be as small as possible (efficiency). To realize
these requirements, an adaptive quantum estimation (AQE)
was proposed, and recently was proved to have the strong
consistency and asymptotic efficiency.
In the presentation, we will report the first experimental
demonstration of AQE. The angle of a half wave plate that
initializes the linear polarization of input photons is
estimated using AQE. The statistical analysis of these results
verifies the strong consistency and asymptotic efficiency
of AQE. It is expected that AQE will provide a useful methodology
in the broad area of quantum information processing, communication,
and metrology.